Many oil wells have submersible centrifugal pumps for pumping well fluid. A motor operatively couples to the pump to drive the pump. The pump has a large number of stages, each stage having an impeller and a diffuser. The impeller has impeller flow passages that extend upward and outward to propel fluid into diffuser flow passages extending upward and inward.
The impellers are free to move axially a short distance relative to the diffusers in response to downthrust and upthrust imposed on the impellers. Downthrust acts in an upstream direction on the impeller, while upthrust acts in a downstream direction on the impeller. Each diffuser has a downward facing upthrust surface located above an upward facing upthrust surface on the impeller. An upthrust washer located between the diffuser upthrust surface and the impeller upthrust surface transfers the upthrust from the impeller to the diffuser. Impellers and diffusers also have downthrust surfaces and downthrust washers.
A balance ring on an upper side of the impeller is in rotating engagement with a cavity wall on the lower side of the diffuser. Some of the well fluid discharged from the impeller passages escapes into the diffuser cavity above the impeller through a small clearance between the balance ring and the cavity wall. The fluid that enters the diffuser cavity is normally at a higher pressure than the fluid within the impeller passages, creating a higher pressure zone in the diffuser cavity that acts on the impeller. Many impellers have balance holes extending from the upper side of the impeller into the impeller flow passages in communication with the well fluid flowing into the diffuser cavity. The balance holes allow some of the higher pressure fluid in the diffuser cavity to flow or drain through the balance holes into the impeller passages, reducing the pressure in the diffuser cavity.
In some designs the upthrust washer will be located above the inlets of the balance holes. During full upthrust, the upthrust washer will be squeezed between the impeller upthrust surface and the diffuser upthrust surface. Thus, while the impeller is in a full upthrust condition, the upthrust washer will block flow from the diffuser cavity down through the balance holes.